The present invention relates to a method, for use with a radar, for detecting a stationary on-road object other than a vehicle, for example, an object that has fallen off a traveling vehicle.
For vehicle-to-vehicle distance control, a vehicle-mounted radar system, which projects a radar beam forward and thereby detects an object such as a vehicle located in the path ahead, is used. Such a radar system can be an FM-CW radar system that uses radio waves such as millimeter waves or a system that use laser light. Using such a radar system, the distance and relative velocity with respect to the vehicle ahead and the accurate position of the vehicle ahead are detected to control the vehicle-to-vehicle distance.
A scanning radar scans a radar beam by moving the beam from left to right or from right to left, with small step angles, within a predetermined time. At each step angle, the radar-equipped vehicle projects a radar beam toward the vehicle traveling on the road ahead, and the reflected wave from the vehicle ahead is received and processed to detect the presence of the vehicle ahead and compute the distance and relative velocity with respect to that vehicle.
In the prior art, the target detected by the radar is examined to determine whether it is a vehicle traveling ahead or a structure such as a bridge or a road sign located above the road ahead, and when it is a vehicle traveling ahead, the detected target is set as a vehicle control target.
On the other hand, if the detected target is a stationary object, it may be a stationary object lying on the road ahead. There are various kinds of stationary objects that may be lying on the road; for example, the object may be an object, such as a cardboard box, that has fallen off a truck or a roadside billboard blown down by the wind. Such stationary on-road objects vary in size and shape; some are small enough that a vehicle can pass over them, and others are not. However, there are no set criteria for assessing the size, shape, etc. of such objects, and even when the stationary object located on the road ahead is one that a vehicle can pass over, the object is recognized as a vehicle control target, and control such as deceleration is performed, for example, in the case of vehicle-to-vehicle-control. One solution to this would be to exclude such stationary objects from control targets so that any of such targets would not be detected as a target, but there are stationary on-road objects that a vehicle cannot pass over; in that case, they cannot be excluded from the control targets.
It is accordingly an object of the present invention to provide a method for determining whether an object detected by a radar is a stationary on-road object, that is, the kind of on-road object that a vehicle cannot pass over. A scanning type radar or a non-scanning type radar may be used as the radar in the present invention.
According to a stationary on-road object detection method for a radar pursuant to the present invention, a fluctuation in the reception level of a reflected wave from a target is obtained in relation to the distance of the target, a difference in reception level between a maximum point and a minimum point is obtained from the fluctuation of the reception level, and when the obtained difference is larger than a predetermined threshold value, it is determined that the target is a stationary on-road object. In an alternative method, the slope over the distance between the maximum point and the minimum point is obtained, and when the obtained slope is greater than a predetermined threshold value, it is determined that the target is a stationary on-road object. In another alternative method, the distance between maximum points or between minimum points is obtained, and when the obtained distance is smaller than a predetermined threshold value, it is determined that the target is a stationary on-road object. The above detection methods may be combined.
Provisions are made to vary the threshold value for the difference and the threshold value for the slope in accordance with the distance of the target. Further, for the reception level, a filter constant is varied in accordance with relative velocity or actual vehicle speed.
A scanning type radar or a non-scanning type radar may be used as the radar in the present invention. In the description hereinafter given, an FM-CW radar is used, but the radar in the invention is not limited to the FM-CW radar.
According to the present invention, whether the detected target is a stationary on-road object that a vehicle cannot pass over can be determined based on the fluctuation of the reception level by using a simple method. This determination can be made based on the difference in value between a maximum point and a minimum point, the slope, or the distance between maximum points or between minimum points; an appropriate one of these methods can be selected. It is also possible to combine these methods; furthermore, as the threshold value is varied in accordance with the distance or relative velocity of the target, a more accurate determination can be made.